In recent years a number of auto focus systems particularly for use with cameras have been devised. The majority of such auto focus systems fall into one of two main types: first the passive type systems wherein two images of a scene being viewed are compared with the amount of displacement from a coincidence or superimposed position being indicative of the range to the subject and second, the active type systems wherein a projection of either sound or light is directed from the camera to the subject and the reflected energy received back is analyzed to determine the distance to the subject. The present invention relates to an active type system which, in the past, have encountered several difficulties.
Active units, using sound as the projection beam, suffer the problems of reflections off of objects which are not the main subject of the picture and the inability to focus through a transparent medium such as a window. Active systems using light or infrared energy heretofore have usually required moveable projections and/or moveable detectors or have needed multiple projectors to establish a focus position. In some systems, a fixed projector and fixed detectors have been employed but these systems require specially shaped or masked detectors and/or use rather complex electronics to determine the position of the reflected light. Furthermore, prior art systems have produced primarily analog output signals which are difficult to process and use for positioning a camera lens. While steps have been taken to overcome most of the problems encountered with prior art systems and accurate in-focus pictures may be obtained in a majority of the cases with either type system, a truly simple system having a digital output, having no moving parts other than the camera taking lens, having simple electronics, and having a low manufacturing cost has yet to be devised.